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It is often accepted that various factors modulate ageing and longevity in a linear way, a higher/lower level of the factor delaying ageing and/or increasing longevity. However, many examples (e.g. the effects of diet restriction or antioxidants and of telomere attrition) show that this view can be wrong. For instance, mild stress has often positive effects on ageing and longevity, and severe stress opposite effects. The dose-response relationship is thus not linear but has an inverted U-shape. Therefore, in many cases, the concept of factors modulating ageing and longevity in a linear way should be rejected, and this has consequences for physicians' guidance.